This invention relates to cranes. As used herein, the term "crane" is intended to include like lifting machines, such as hoists.
Furthermore, as used herein, the term "crane hook" is intended to include like load-suspension devices, such as shackles.
A problem exists where cranes operate in unstable environmental conditions. For example, where the crane is rising and falling relative to the load to be handled by the crane, as occurs where the crane is mounted on an off-shore oil rig and the load is carried on a vessel disposed alongside the rig. Another example occurs when the load to be lifted is again carried by a vessel but in this case the crane (which may now be in the form of a hoist) is mounted on a fixed or rotary wing aircraft.
The problem stems from the need to lower the crane hook (or the like) carefully to a position adjacent the load and then to attach the hook to the load by way of slings, strops or the like, already placed in position. This operation has to be carried out quickly and at an opportune moment, before relative movement between the load and the crane makes attachment to the hook impossible or dangerous.
It will be appreciated that the moment of attachment must be judged precisely; the time available for attachment may be very limited and once attachment is made, the load must be lifted immediately. Furthermore, it will be appreciated that personnel making the attachment may have little or no time to stand clear.
Successful operations are therefore achieved only by the skill of the personnel employed, but, nevertheless, they are potentially unsafe.